As a conventional typical example of such a flow meter device of the fluid, there is a flow meter device shown in FIG. 11 (e.g., see Patent Literature 1).
This flow meter device includes a first ultrasonic transducer 32 attached to a fluid passage 31 through which the fluid flows, a second ultrasonic transducer 33 attached to the fluid passage 31, a switching means 34 which performs switching between transmission and reception of the first ultrasonic transducer 32 and the second ultrasonic transducer 33, a transmission means 35 which activates the first ultrasonic transducer 32 and the second ultrasonic transducer 33, an amplification means 36 which amplifies a signal which has been received by the ultrasonic transducer at a reception side and has passed through the switching means 34 to a predetermined amplitude level, and a reference voltage comparison means 37 which compares the voltage of the received signal which has been amplified by the amplification means 36 to a reference voltage.
As shown in FIG. 12, the reference voltage comparison means 37 compares the amplified received signal A to the reference voltage, and outputs a signal C from a timing c at which the magnitude relation between the amplified received signal A and the reference voltage is inverted to a zero cross point a that comes first after the timing c, and a determination means 38 outputs a signal D to a repeating means 39 at the zero cross point a.
The repeating means 39 counts the signal received from the determination means 38 at preset number of times, and outputs the signal received from the determination means 38 to a control means 42. A time measuring means 40 measures time for which the repeating means 39 has counted the preset number of times. A flow rate calculation means 41 calculates a flow rate based on the time measured by the time measuring means 40.
The control means 42 is configured to control the operation of the transmission means 35 based on the flow rate calculated by the flow rate calculation means 41 and output to the control means 42 and the signal output from the repeating means 39.
In this configuration, the control means 42 causes the transmission means 35 to operate and the ultrasonic transducer 32 to transmit an ultrasonic signal. Then, the ultrasonic signal propagates (travels) through the flow and is received by the second ultrasonic transducer 33. The ultrasonic signal is amplified by the amplification means 36, and then processed by the reference voltage comparison means 37 and the determination means 38. Then, the ultrasonic signal is input to the control means 42 through the repeating means 39. The above described operation is repeated preset n times, and the time measuring means 40 measures the time for the repeated operation.
The same operation is performed while performing switching between transmission and reception of the first ultrasonic transducer 32 and the second ultrasonic transducer 33, by the switching means 34. Transit time (propagation time) for which the ultrasonic signal propagates through a measurement target fluid from an upstream side to a downstream side (this direction will be referred to as a forward flow direction), and transit time for which the ultrasonic signal propagates through the measurement target fluid from the downstream side to the upstream side (this direction will be referred to as a reverse flow direction) are respectively measured, the flow velocity of the measurement target fluid is derived, and a flow rate Q is derived according to a formula (1).
When the effective distance between the ultrasonic transducers in a flow direction is L, measurement time that takes for the ultrasonic signal to propagate from the upstream side to the downstream side n times is t1, measurement time that takes for the ultrasonic signal to propagate from the downstream side to the upstream side n times is t2, the flow velocity of the measurement target fluid is v, the cross-sectional area of the fluid passage is S, and the angle formed between the propagation path of the ultrasonic wave between the ultrasonic transducers and the flow of the measurement target fluid is φ, the flow rate Q of the measurement target fluid is represented by the following formula:Q=S·v=S·L/2·cos φ((n/t1)−(n/t2))  (formula 1)
Actually, the flow rate is calculated by multiplying the formula 1 by a coefficient corresponding to the flow rate.
The gain of the amplification means 36 is adjusted so that the signal received by the ultrasonic transducer at the reception side has a constant amplitude. The gain is adjusted so that the peak voltage value of the received signal falls within a predetermined voltage range.
The above described flow meter device of the fluid is electrically activated by batteries, and required to reduce electric power consumption. It is considered that in this conventional flow meter device of the fluid, a reference voltage is newly set (e.g., see Patent Literature 2).